Just as there are marked differences in the structure and composition of hard contact lenses, cellulose acetate butyrate (CAB) and silicone copolymer gas-permeable contact lenses and soft contact lenses, there are also marked differences in the maintenance and care or treatment with the various types of hard, CAB, silicone and soft lenses. While patient care and treatment of hard contact or conventional contact lenses is relatively simple and uncomplicated, the proper care and treatment of CAB and silicone copolymer lenses (gas-permeable) and the newer soft and hydrophilic lenses has proved to be more complex, time consuming and costly to the patient.
The primary difference between the conventional hard contact lens and the silicone copolymer lenses and the more complex soft lenses is the hydrophobic nature of the silicone copolymer lenses and the marked increase in the polar or water attracting centers of the hydrophilic gel material from which the soft contact lenses are made. It is this property of the hydrophilic gel lens that gives the soft lens its own unique physical properties and clinical behavior. This polar or water attracting center of the gel material is represented in the hydroxyethyl methacrylate bond as a hydroxyl group (--OH) which attracts and holds large amounts of water. It is this high water content held in the expanded matrix of the hydrophilic gel lens which leads to the special difficulties in cleaning and disinfecting or asepticising the soft hydrophilic lens. The hydrophilic nature of soft contact lenses makes the lenses vulnerable to bacterial contamination. While studies have demonstrated that bacteria cannot penetrate the actual intromolecular pores of the hydrophilic lens, except in defective lenses, the bacteria have an affinity for protein and tear deposits on the surfaces of the lens matrix. In particular, the tears and fluids absorbed by the soft lenses serve as excellent bacterial culture media. If defects or nicks occur in the lenses either during manufacture or subsequent patient wear, bacteria may find a haven to grow and be sheltered from superficial lens cleaning and disinfection.
Potentially harmful fungi are also a possible danger to the soft contact lens. Fungi, like bacteria, can thrive in tear secretions, other fluids or deposits and penetrate the lens material directly if enzymatic degradation of the lens material has taken place.
Similarly, any substantial residual proteinaceous or tear secretion deposits or lipid deposits remaining in or on the lens may readily overwhelm and inactivate the most effective germicidal components of a disinfecting system, and may thus serve to act as a growth media for a variety of potentially harmful microorganisms and fungi. Therefore, it is important that prior to storing the soft contact lenses in a disinfecting solution, protein and lipid deposits be removed from the lens surfaces so that the disinfectant properties of the sterilizing solution or method will not be overwhelmed by gross organic or inorganic deposits and pollutants. Therefore, an effective cleaning step or steps is an essential and mandatory part of any effective soft lens treatment and maintenance regimen.
Wetting solutions are used to prepare the contact lenses prior to insertion into the eye and are known in the prior art. Prior art contact lens wetting solutions have primarily involved the use of polyvinyl alcohol as a wetting agent and methyl cellulose or hydroxyethyl cellulose as viscosity building agent. These prior art solutions have also contained sufficient amounts of water-soluble salts, generally sodium chloride, to make them isotonic or hypertonic with human serum and tear fluid. For example, hypertonic wetting solutions are disclosed in U.S. Pat. No. 3,549,747.
Re-wetting solutions are instilled directly into the eyes when contact lenses are being worn. Such solutions can also be used before or after wearing periods. The purpose of re-wetting solutions include providing comfort and relubrication for the eye.
Because of the potential for bacterial contamination, wetting and re-wetting solutions generally include a preservative system to prevent or inhibit microbial growth, especially where multi-dose containers of the solution are prepared. Generally, where no preservative is employed, single dose containers are utilized, which result in greater expense.
Preservative systems known in the prior art for wetting solutions are disclosed in U.S. Pat. No. 3,549,747. Known preservatives include benzalkonium chloride, thimerosal, chlorhexidine, sorbic acid and potassium sorbate. However, these compounds have drawbacks in that they can be concentrated in the lens matrix and cause irritation, excessive burning, red eye and lens discoloration, which can prevent the patient from wearing the lenses or can otherwise be hypersensitive and annoying to the eye.
With the advent of extended wear contact lenses, it becomes even more important to avoid such problems, since those lenses can remain in the eye for several weeks. Thus, a need has arisen for an effective preservative system which avoids the use of preservatives known to cause eye irritation and lens discoloration and which is suitable for soft contact lenses in a wetting and rewetting solution. A need also exists for a soft contact lens wetting and re-wetting solution which incorporates such a preservative system. Finally, a need exists for an "in-eye" re-wetting solution that can be instilled directly into the eyes to provide re-lubrication and comfort for the eyes having such a preservative system.